Transparent SSL TLS interception: SSLsplit

Transparent SSL TLS interception: SSLsplit is a tool for man-in-the-middle attacks against SSL/TLS encrypted network connections. It is intended to be useful for network forensics, application security analysis, web application security testing, network security auditing, penetration testing and any other network or security testing where encrypted SSL or TLS connection is being established. Connections are transparently intercepted through a network address translation engine and redirected to SSLsplit. SSLsplit terminates SSL/TLS and initiates a new SSL/TLS connection to the original destination address, while logging all data that is being transmitted through encrypted network connection.

_ Transparent and scalable SSL TLS interception v0.5.0 Released _

SSLsplit is designed to transparently terminate connections that are redirected to it using a network address translation engine. SSLsplit terminates SSL/TLS and initiates a new SSL/TLS connection to the original destination address, while logging all data transmitted. Besides NAT based operation, SSLsplit also supports static destinations and using the server name indicated by SNI as upstream destination. SSLsplit is purely a transparent proxy and cannot act as a HTTP or SOCKS proxy configured in a browser.

SSLsplit supports plain TCP, plain SSL, HTTP and HTTPS connections over both IPv4 and IPv6. SSLsplit fully supports Server Name Indication (SNI) and is able to work with RSA, DSA and ECDSA keys and DHE and ECDHE cipher suites. Depending on the version of OpenSSL built against, SSLsplit supports SSL 3.0, TLS 1.0, TLS 1.1 and TLS 1.2, and optionally SSL 2.0 as well.

For SSL and HTTPS connections, SSLsplit generates and signs forged X509v3 certificates on-the-fly, mimicking the original server certificate’s subject DN, subjectAltName extension and other characteristics. SSLsplit has the ability to use existing certificates of which the private key is available, instead of generating forged ones. SSLsplit supports NULL-prefix CN certificates but otherwise does not implement exploits against specific certificate verification vulnerabilities in SSL/TLS stacks.

SSLsplit implements a number of defences against mechanisms which would normally prevent MitM attacks or make them more difficult. SSLsplit can deny OCSP requests in a generic way. For HTTP and HTTPS connections, SSLsplit removes response headers for HPKP in order to prevent public key pinning, for HSTS to allow the user to accept untrusted certificates, and Alternate Protocols to prevent switching to QUIC/SPDY. HTTP compression, encodings and keep-alive are disabled to make the logs more readable.

As an experimental feature, SSLsplit supports STARTTLS and similar mechanisms, where a protocol starts on a plain text TCP connection and is later upgraded to SSL/TLS through protocol-specific means, such as the STARTTLS command in SMTP. SSLsplit supports generic upgrading of TCP connections to SSL.

See the manual page sslsplit for details on using SSLsplit and setting up the various NAT engines.

So in order for attack to be successful attacker has to be placed somewhere in between the victim machine and the server. This can be done in many different ways:

• Use ARP spoofing to redirect the traffic of the victim by publishing false mappings from the standard gateway MAC address to the attacker’s IP address. You do not need physical access to the victim’s device to do that. Check out the arpspoof tool.
• Change the default gateway address in the victim’s network settings. This is the easiest method if you have access to the victim’s device.
• Forging DNS entries with a DNS server that returns the attacker’s IP address for certain (or all) domains.
• Redirect traffic for individual domains by modifying entries in the /etc/hosts file of the victim’s machine.
• …

The easiest way is to change the default gateway address of your victim’s device to the attacker’s IP address. That makes sure that all the traffic goes through your machine.

• SSLsplit supports plain TCP, plain SSL, HTTP and HTTPS connections over both IPv4 and IPv6.
• For SSL and HTTPS connections, SSLsplit generates and signs forged X509v3 certificates on-the-fly, based on the original server certificate subject DN and subjectAltName extension.
• SSLsplit fully supports Server Name Indication (SNI) and is able to work with RSA, DSA and ECDSA keys and DHE and ECDHE cipher suites.
• SSLsplit can also use existing certificates of which the private key is available, instead of generating forged ones.
• SSLsplit supports NULL-prefix CN certificates and can deny OCSP requests in a generic way.
• SSLsplit removes HPKP response headers in order to prevent public key pinning.

Recommended Reading

Requirements for the transparent SSL TLS interception:

SSLsplit depends on the OpenSSL and libevent 2.x libraries. The build depends on GNU make and a POSIX.2 environment in PATH . The (optional) unit tests depend on the check library.

SSLsplit currently supports the following operating systems and NAT engines:

• FreeBSD: pf rdr and divert-to, ipfw fwd, ipfilter rdr
• OpenBSD: pf rdr-to and divert-to
• Linux: netfilter REDIRECT and TPROXY
• Mac OS X: ipfw fwd and pf rdr (experimental)

Installation

SSLsplit is available as a package or port on the following systems, with varying up-to-dateness:

• FreeBSD: security/sslsplit
• OpenBSD: security/sslsplit
• NetBSD: security/sslsplit
• DragonFly BSD: security/sslsplit
• Mac OS X ~~ Homebrew: sslsplit ~~
• Arch Linux: sslsplit
• ALT Linux autoimports: sslsplit
• Debian: sslsplit
• Ubuntu: sslsplit
• Fedora/RHEL/CentOS: sslsplit
• Mageia: sslsplit
• ~~ ArchAssault: sslsplit ~~
• BlackArch Linux: sslsplit
• Kali Linux: sslsplit
• ~~ Backtrack: sslsplit ~~
• WiFi Pineapple Infusion: sslsplit

To install from source:

make
make test       # optional unit tests
make install    # optional install

Usage

[youtube url=”qzNv6e1z1_E” fs=”1″ hd=”1″ width=”100%” height=”400″]

% sslsplit -h
Usage: sslsplit [options...] [proxyspecs...]
  -c pemfile  use CA cert (and key) from pemfile to sign forged certs
  -k pemfile  use CA key (and cert) from pemfile to sign forged certs
  -C pemfile  use CA chain from pemfile (intermediate and root CA certs)
  -K pemfile  use key from pemfile for leaf certs (default: generate)
  -t certdir  use cert+chain+key PEM files from certdir to target all sites
              matching the common names (non-matching: generate if CA)
  -O          deny all OCSP requests on all proxyspecs
  -P          passthrough SSL connections if they cannot be split because of
              client cert auth or no matching cert and no CA (default: drop)
  -g pemfile  use DH group params from pemfile (default: keyfiles or auto)
  -G curve    use ECDH named curve (default: secp160r2 for non-RSA leafkey)
  -Z          disable SSL/TLS compression on all connections
  -s ciphers  use the given OpenSSL cipher suite spec (default: ALL:-aNULL)
  -e engine   specify default NAT engine to use (default: ipfw)
  -E          list available NAT engines and exit
  -u user     drop privileges to user (default if run as root: nobody)
  -j jaildir  chroot() to jaildir (default if run as root: /var/empty)
  -p pidfile  write pid to pidfile (default: no pid file)
  -l logfile  connect log: log one line summary per connection to logfile
  -L logfile  content log: full data to file or named pipe (excludes -S)
  -S logdir   content log: full data to separate files in dir (excludes -L)
  -d          daemon mode: run in background, log error messages to syslog
  -D          debug mode: run in foreground, log debug messages on stderr
  -V          print version information and exit
  -h          print usage information and exit
  proxyspec = type listenaddr+port [natengine|targetaddr+port|"sni"+port]
  e.g.        http 0.0.0.0 8080 www.roe.ch 80  # http/4; static hostname dst
              https ::1 8443 2001:db8::1 443   # https/6; static address dst
              https 127.0.0.1 9443 sni 443     # https/4; SNI DNS lookups
              tcp 127.0.0.1 10025              # tcp/4; default NAT engine
              ssl 2001:db8::2 9999 pf          # ssl/6; NAT engine 'pf'
Example:
  sslsplit -k ca.key -c ca.pem -P  https 127.0.0.1 8443  https ::1 8443

Further Reading and Stories

These links are provided in the hope that they may be useful, but without implying endorsement of any kind.

• Use SSLsplit to transparently sniff TLS/SSL connections – including non-HTTP(S) protocols by Philipp C. Heckel
• ARP spoofing, MITM attack over SSL-splitted connections, and fake CA certificate forging. All together. Against an iPhone by Claudio Di Nardo
• How to Create a Transparent SSL Forward Proxy using SSLsplit on CentOS 7.x to Capture/Sniff SSL packets by Richard Yau
• Intercepting SSL And HTTPS Traffic With mitmproxy and SSLsplit by Vladimir Zakharevich and Martin Rakhmanov
• Exploiting the Superfish certificate by Robert Graham
• Exploiting the Superfish certificate, the OpenWRT way by AO Itsuugo
• Yik Hak: Smashing the Yak by Sanford Moskowitz
• Fun with honeypots by Eric Severance
• Threema security assessment by Hristo Dimitrov, Jan Laan and Guido Pineda
• sslsplit on Windows by Piotr Hałaczkiewicz
• SSLSplit (sic) unter Kali-Linux einrichten by magnus at cryto dot org
• SSL-Traffic mit SSLsplit und Wireshark untersuchen by Andreas Hainke
• Herramientas para realizar MITM (man-in-the-middle) en SLL (sic) by Álvaro Paz
• SSL中间人证书攻击测试演练 by xia0k
• 用SSLsplit分析SSL连接-包括非https协议 by Zhiwei Li

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